You could adjust the near clip-plane to get the culling…I think
Update: It looks like UE4 does not support custom NearClip Plane for SceneCapture2D component or cameras. UE3 had this feature.
I have created a demo by rotating the scenecapture2d component and adjusting its FOV angle based on distance. The results are far from ideal, but will give the player an illusion of a portal as long as he is not too close to it.